Weight Fluctuations with High Intensity Exercise

I have noticed a consistent pattern for my weight, which is this: after intense and/or long workouts or races, I usually see an increase. This was true for this past weekend--yesterday morning, my weight was up by 2 pounds. The longer the workout/event, the more likely it seems that my weight will increase. Sometimes the weight comes off but sometimes not and currently I am stuck in this weekly two- to 4-pound fluctuation. It is a source of frustration to me because it seems like exercising 20 hours per week should make weight loss relatively easy. And seriously, after spending 11 hours exercising in 2 days, don't you think I would weigh less the next day, not 2 pounds more? Since my exercise load has increased, it has also become more difficult to lose weight. I think part of the explanation is more hypoglycemia but that can't explain it all. I try to take solace in knowing that my body is incredibly efficient and doesn't waste a single calorie...